Nephrogenic systemic fibrosis is associated with hypophosphataemia: a case–control study

Abstract

Objective. Nephrogenic systemic fibrosis (NSF) is an iatrogenic fibrosing disorder that primarily affects individuals with chronic kidney disease (CKD) following exposure to gadolinium-based contrast agents (GBCAs). Derangements of calcium and phosphorus have been reported in patients with NSF. The aim of this study was to investigate potential factors in addition to GBCA exposure that may be involved in the pathogenesis of NSF. We hypothesized that patients with stage 5 CKD and NSF would manifest greater alterations in calcium, phosphorus and fibroblast growth factor 23 (FGF23) levels than those who do not have NSF.

Methods. Levels of phosphorus, calcium, FGF23 and 25-hydroxy-vitamin D were measured in 10 patients with stage 5 CKD and biopsy-proven NSF and in 19 patients with stage 5 CKD without NSF. Statistical analyses were performed using Fisher’s exact test for categorical variables and the Kruskal–Wallis test for continuous variables.

Results. Patients with NSF had significantly lower phosphorus levels compared with controls (P = 0.01). There were no significant differences between NSF patients and controls in calcium, 25-hydroxy-vitamin D, intact parathyroid hormone or FGF23 levels.

Conclusion. Differences in phosphorus metabolism may exist between patients with stage 5 CKD and NSF compared with patients with stage 5 CKD without NSF.

nephrogenic systemic fibrosis, gadolinium, phosphorus, fibroblast growth factor 23, magnetic resonance imaging, chronic kidney disease, dialysis

Introduction

Nephrogenic systemic fibrosis (NSF) is an iatrogenic fibrosing disorder that primarily affects individuals with chronic kidney disease (CKD) following exposure to gadolinium-based contrast agents (GBCAs) during imaging procedures. NSF is a disabling and often painful condition that is characterized by skin hardening, tethering, and hyperpigmentation, often with cobblestoning, flexion contractures of the joints and extracutaneous fibrosis [1]. Given that not all patients with CKD who are exposed to GBCAs develop NSF, additional factors are probably involved in its pathogenesis.

Derangements of calcium and phosphorus have been reported in patients with NSF. Marckmann et al. [2] observed significantly elevated levels of ionized calcium and phosphorus among NSF cases at the time of gadodiamide exposure as compared with controls with stage 5 CKD but without NSF. Abraham et al. [3] found deposits of calcium and phosphorus in gadolinium-containing skin biopsy samples from patients with NSF.

Fibroblast growth factor 23 (FGF23) is a phosphorus- and vitamin D–regulating hormone, levels of which increase markedly as glomerular filtration rate decreases [4–6]. In addition to its effects on phosphorus and vitamin D homeostasis, recent studies have demonstrated that FGF23 can activate FGF receptors in non-classical target cells [7]. In subtotal nephrectomized rats (5/6) that had received five daily injections of 2.5 mmol/kg of gadodiamide, FGF23 levels were significantly higher 11 days after the last injection than in rats that had received five daily injections of saline [8]. Whether markedly elevated FGF23 levels in patients with stage 5 CKD can activate dermal fibroblasts and thereby contribute to the development of NSF is unknown.

The aim of this study was to investigate potential factors in addition to GBCA exposure that may be involved in the pathogenesis of NSF. We hypothesized that patients with stage 5 CKD and NSF would manifest greater alterations in calcium, phosphorus and FGF23 levels than those who do not have NSF.

Patients and methods

Study design

In this case–control study, patients with stage 5 CKD and NSF were compared with patients with stage 5 CKD without NSF. Participants were recruited from the outpatient nephrology and rheumatology practices at Massachusetts General Hospital. Twenty-nine patients ≥18 years of age with stage 5 CKD undergoing haemodialysis or peritoneal dialysis were enrolled between February 2009 and February 2010. Cases consisted of 10 patients with NSF following GBCA exposure, confirmed by histological examination of a skin biopsy. Controls consisted of 19 patients with stage 5 CKD who did not have NSF: 9 had never been exposed to a GBCA (control group A) and 10 had previously received a GBCA (control group B). Cases and controls were matched for age and sex. Of the 20 patients (10 cases and 10 group B controls) who had previously received a GBCA, 5 had received gadopentetate dimeglumine and 2 had received gadodiamide; the type of GBCA was not documented for the remaining 13. Blood was collected from all 29 patients. Outcome measures of interest were levels of plasma calcium, plasma phosphorus, plasma FGF23, serum 25-hydroxy-vitamin D and intact plasma parathyroid hormone (PTH). Written informed consent was obtained from all participants according to the Declaration of Helsinki. This study was approved by the Partners Institutional Review Board of Massachusetts General Hospital.

Assays

Plasma FGF23 levels were measured in duplicate using a second-generation C-terminal assay (Immutopics, San Clemente, CA, USA) with a mean intra-assay coefficient of variation of <5%. The upper limit of detection for this assay is 1500 reference units (RU)/ml. Samples with FGF23 levels >1500 RU/ml required serial dilutions to obtain a quantitative measurement. Measurement of plasma calcium, plasma phosphorus, serum albumin, intact plasma PTH and serum 25-hydroxy-vitamin D was performed by local laboratories using standard procedures.

Statistical analyses

Statistical analyses were performed using Fisher’s exact test for categorical variables and the Kruskal–Wallis test for continuous variables. All analyses were performed using SAS version 9.3 (SAS Institute, Cary, NC, USA).

Results

Baseline characteristics of cases and controls are listed in Table 1. Patients were predominantly male and the majority identified themselves as Caucasian. Mean age did not differ between cases and the control groups (P = 0.95). There was a difference between cases and controls in modality of dialysis: all NSF patients compared with 11% of control group A and 50% of control group B patients were receiving haemodialysis (P < 0.001). The remainder of the controls were receiving peritoneal dialysis. The cumulative GBCA dose was similar between NSF patients and patients in control group B (P = 0.16). A similar proportion of NSF cases and patients in control group B were receiving erythropoiesis-stimulating agents; however, a greater proportion of patients in control group A were receiving erythropoiesis-stimulating agents than either NSF cases or patients in control group B (P = 0.03). There were no significant differences between the groups in history of renal transplantation, parathyroidectomy, diabetes mellitus, coronary artery disease, cerebrovascular disease or smoking or in use of phosphate binders or cinacalcet.

Table 1

Baseline characteristics

	NSF patients (n = 10)	Controls who never received GBCA (control group A; n = 9)	Controls who received GBCA (control group B; n = 10)	P-value
Age, mean (s.d.), years	62.4 (14.2)	64.3 (10.7)	62.5 (11.1)	0.95
Female, n (%)	3 (30)	3 (33)	5 (50)	0.71
Race, n (%)
White	9 (90)	7 (78)	7 (70)	0.65
Black	1 (10)	1 (11)	1 (10)
Asian, Pacific Islander	0	1 (11)	0
Other	0	0	2 (20)
Ethnicity, n (%)
Hispanic	0	0	2 (20)
Non-Hispanic	9 (90)	8 (89)	8 (80)
Cumulative GBCA dose, median (Q1–Q3), ml	60 (60–135)		47.5 (40–80)	0.16
Modality of dialysis, n (%)				0.0003
Haemodialysis	10 (100)	1 (11)	5 (50)
Peritoneal dialysis	0	8 (89)	5 (50)
Aetiology of renal failure, n (%)				0.98
Alport’s syndrome	1 (10)	0	0
Diabetes mellitus	4 (40)	3 (33)	2 (20)
Hypertension	2 (20)	1 (11)	1 (10)
Focal segmental glomerulosclerosis	1 (10)	1 (11)	3 (30)
Granulomatosis with polyangiitis	1 (10)	0	1 (10)
Obstructive uropathy	1 (10)	1 (11)	1 (10)
Polycystic kidney disease	0	1 (11)	1 (10)
IgA nephropathy	0	0	1 (10
Henoch–Schönlein purpura	0	1 (11)	0
Fibrillary glomerulonephritis	0	1 (11)	0
History of renal transplantation, n (%)	4 (40)	0	1 (10)	0.09
History of parathyroidectomy, n (%)	3 (30)	0	1 (10)	0.29
Phosphate binder use, n (%)	8 (80)	7 (78)	10 (100)	0.07
Cinacalcet use, n (%)	1 (10)	1 (11)	2 (20)	>0.99
Erythropoiesis-stimulating agent use, n (%)	2 (20)	7 (78)	3 (30)	0.03
Co-morbidities, n (%)
Diabetes mellitus	5 (50)	4 (44)	2 (20)	0.40
Coronary artery disease	7 (70)	2 (22)	3 (30)	0.11
Cerebrovascular disease	4 (40)	0	2 (20)	0.12
Ever smoker	6 (60)	5 (56)	8 (80)	0.55

GBCA: gadolinium-based contrast agent; NSF: nephrogenic systemic fibrosis; Q1–Q3: quartile 1–quartile 3.

Patients with NSF had significantly lower phosphorus levels compared with both control groups (P = 0.01) (Table 2). Adjusting for the use of phosphate binders via analysis of variance did not alter this finding. There were no significant differences between NSF patients and controls in calcium, 25-hydroxy-vitamin D, intact PTH or FGF23 levels.

Table 2

Laboratory values

	NSF patients (n = 10)	Controls who never received GBCA (control group A; n = 9)	Controls who did receive GBCA (control group B; n = 10)	P-value
Phosphorus, mean (s.d.), mg/dl	3.4 (0.87)	4.58 (1.01)	4.41 (1.14)	0.01
Albumin, mean (s.d.), g/dl	4.04 (0.68)	4.16 (0.34)	4.06 (0.29)	0.88
Calcium, mean (s.d.), mg/dl	9.53 (0.92)	9.67 (0.61)	9.61 (0.58)	0.89
25-hydroxy-vitamin D, mean (s.d.), ng/ml	29.53 (23.90)	25.38 (13.82)	28.42 (16.78)	0.97
Parathyroid hormonea, mean (s.d.), pg/ml	267.4 (171.7)	255.3 (100.1)	305.1 (210.3)	0.83
FGF23, median (Q1–Q3), RU/ml	6744 (2984–10 679)	6959 (4446–10 809)	6228 (2719–12 022)	0.76

^aPTH values were available for 7 of the 10 cases, 6 of the 9 group A controls and 7 of the 10 group B controls. GBCA: gadolinium-based contrast agent; NSF: nephrogenic systemic fibrosis; FGF23: fibroblast growth factor 23; Q1–Q3: quartile 1–quartile 3; RU: reference units.

Discussion

In this case–control study, patients with NSF and stage 5 CKD had significantly lower phosphorus levels than controls with stage 5 CKD who did not have NSF. However, calcium, 25-hydroxy-vitamin D, intact PTH and FGF23 levels were similar between the NSF patients and controls.

Several reasons may explain why the significantly lower phosphorus levels observed among patients with stage 5 CKD and NSF compared with those without NSF cannot be accounted for by differences in dialysis modality, potential differences in dietary phosphorus intake or potential differences in renal function. Although all of the NSF patients were receiving haemodialysis, whereas the majority of the controls were undergoing peritoneal dialysis, FGF23 levels do not differ significantly between patients undergoing haemodialysis and those undergoing peritoneal dialysis [9]. It is possible that the NSF patients were sicker and had less oral intake than the controls, which could result in lower phosphorus levels among the NSF patients. However, the similar albumin levels among NSF patients and controls suggests that nutritional status did not differ between the groups [10]. It is also possible that patients in the control groups consumed greater amounts of phosphorus in their diets, resulting in higher phosphorus levels in the control groups than among patients with NSF. However, better clearance of phosphorus is expected with peritoneal dialysis compared with haemodialysis, given that patients receiving peritoneal dialysis tend to have more residual renal function than those receiving haemodialysis and usually produce some amount of urine. Were potential differences in renal function to have accounted for the difference in phosphorus levels between NSF cases and controls, the NSF cases would be expected to have higher phosphorus levels than the controls, many of whom were receiving peritoneal dialysis; however, this is contrary to what we observed. Therefore the significantly lower phosphorus levels observed among patients with stage 5 CKD and NSF compared with those without NSF cannot be accounted for by potential differences in renal function. Accordingly, there may be intrinsic differences in phosphorus metabolism between patients with stage 5 CKD and NSF and those with stage 5 CKD who do not have NSF.

It is possible that, once released from its chelate, the positively charged free gadolinium complexes with the negatively charged phosphorus and deposits in affected tissues of patients with NSF, thereby leading to lower levels of phosphorus in the circulation of patients with NSF compared with controls. This hypothesis is supported by the observation that at physiologic pH levels, positively charged free gadolinium forms an insoluble precipitate in the presence of phosphate anions [11]. Electron microscopy with energy dispersive X-ray of skin biopsy samples from a Hannover Wistar rat model of NSF has demonstrated peaks indicative of phosphorus [12]. Phosphorus-containing deposits have also been demonstrated in skin biopsies from 20 patients with NSF, also using scanning electron microscopy with energy dispersive X-ray [3]. Given that gadolinium has been identified in skin biopsy samples from patients with NSF [13], it is possible that what is actually depositing in the skin is this insoluble precipitate of gadolinium formed in the presence of phosphate anion.

This study has some limitations. The sample size was small. However, NSF is a rare disease and thus it is necessary to study smaller groups of patients when investigating a relatively uncommon disease. Cases and controls were not matched for modality of dialysis, but FGF23 levels did not differ significantly between the NSF patients, all of whom were receiving haemodialysis, and controls, most of whom were undergoing peritoneal dialysis, suggesting that dialysis modality was not a confounder in this study. Moreover, among all 19 controls, phosphorus levels did not differ between those receiving haemodialysis and those receiving peritoneal dialysis [4.58 mg/dl (s.d. 1.62) vs 4.45 mg/dl (s.d. 0.76), P = 0.80].

Our findings are consistent with the previous literature. Among subtotal nephrectomized rats (5/6), phosphorus levels were significantly lower in those that had received five daily injections of gadodiamide than in those that had received five daily injections of saline. This observation is consistent with our finding that patients with stage 5 CKD and NSF had significantly lower phosphorus levels compared with those with stage 5 CKD without NSF. Further study is warranted to clarify the effects of GBCA compounds on FGF23 and phosphorus metabolism in patients with CKD, especially those receiving dialysis, to see if we can elucidate a particular mechanism for the development of NSF in this susceptible population.

Rheumatology key message

• Nephrogenic systemic fibrosis is associated with hypophosphataemia.

Acknowledgements

We would like to thank all of the patients who participated in this study.

Funding: This work was supported by the Harvard Clinical and Translational Science Center (National Center for Research Resources, National Institutes of Health Award 1UL1RR025758-04). The content is solely the responsibility of the authors and does not necessarily represent the official views of Harvard University and its affiliated academic health care centres or the National Institutes of Health. E.J.B.’s work was supported by an Arthritis Foundation Clinical to Research Transition Award and the National Institutes of Health (grant KL2TR000458).

Disclosure statement: M.S.W. has received personal fees for consultancies with Abbott, Amgen, Sanofi, Luitpold, Pfizer, OPKO, Keryx, Shire and Vifor outside the submitted work and holds stock options in OPKO. All other authors have declared no conflicts of interest.